Interferometry has been a tool for precision measurement long before the invention of lasers. It involves comparing two paths (sample and reference) of single frequency or multi-color light. The result of the phase measurement, as in the Michelson interferometer, is observed as an amplitude modulation of interfering beams. The use of resonator based laser sensors results in a substantial sensitivity improvement, based on their quality factor (“Q”). A physical quantity (e.g., a stress in a fiber or in a waveguide, change in molecular state in a high quality Fabry-Perot, etc) that changes the phase of a resonator, is monitored with a change of its transmission. The sensitivity to the phase change is Q-times higher than the conventional interferometer due to the sharp resonance feature in a resonator, which implies repeated passage through the phase perturbation.
A fiber laser has features that make it very promising for intracavity phase interferometry (IPI) implementation for optical measurement. Its obvious potential advantages over the free space competitors are compactness and good wall-plug efficiency. In addition, Fresnel drag cannot interfere with the measurement if there is no free path in air. There have been preliminary demonstrations that suggest that this approach is practical. Bidirectional ring laser with a beat note response was demonstrated. A fixed carbon nanotube saturable absorber has been implemented to ensured mode-locking and fixed crossing point. A relatively large dead band limits the applicability of this configuration. Improvement of various designs can enhance the accuracy and precision of measurement by IPI.